1. Field of the Invention
The present invention relates to a method for adiabatic cooling type crystallization of organic compound and an apparatus therefore, and more particularly to a method and an apparatus suitable for obtaining paraxylene crystal.
2. Description of the Related Art
Separation and purification of a certain kind of isomer mixture are difficult in distillation operation because boiling points of components constituting the mixture are close to each other. However, there are many cases where melting points are largely different depending on the difference in molecular structures, and therefore, separation by crystallization operation is often effective.
There are methods of extractive and adductive crystallization in which a solvent-agent (extractant, additive) is added to a two-component eutectic system or a multi-component eutectic system as a third component; however, these are disadvantageous to recover the solvent-agent.
In this regard, a method in which a liquified gas component is used as a coolant is advantageous because its recovery is easy.
The present inventors have found that it is effective to carry out crystallization operation, with the use of propane (propene, ethylene, carbon dioxide, ammonia, or the like) as a direct injecting coolant, for a multi-component eutectic system such as xylene mixture (m-xylene+o-xylene+ethylbenzene+p-xylene system) that is a raw material for a typical p-xylene production in a petrochemical industrial process or a xylene mixture (m-xylene+o-xylene+p-xylene system) after isomerization reaction.
In this case, it is possible to carry out the crystallization operation in a jacket type crystallizer; however the crystallization is necessary to be carried out by cooling p-xylene in the multi-component eutectic system to about −30 degrees C. to −60 degrees C. Therefore, it is required to provide the crystallizer with a cooling surface scraper mechanics and a refrigeration unit by which the evaporated coolant from the jacket is compressed by a compressor, for example, under a high pressure of 20 atmospheres, followed by allowing this to be liquified and circulated to the jacket.
Using such a crystallizer results in not only an increase in power cost of the compressor but also increases in facility cost and maintenance cost because the crystallizer has to be provided with a cooling surface scraper mechanics that requires complex and frequent maintenance.
On the other hand, another system in which a heat pump is used is conceivable (Patent document 1: Japanese Patent Application Laid-Open Publication No. 1992-327542), but the system is not necessarily suitable in view of facility cost to construct the heat pump.